The present invention relates to catalytic processes for the preparation of 3-(methylthio)propanal (hereinafter "MMP") and 2-hydroxy-4-(methylthio)butanenitrile ("HMBN"). More particularly, the present invention relates to processes for preparing MMP and HMBN using novel addition catalysts.
MMP and HMBN are intermediates for the manufacture of both d,l-methionine and 2-hydroxy-4-(methylthio)butanoic acid ("HMBA"). Methionine is an essential amino acid commonly deficient in grains used in animal feed compositions. HMBA provides a source of methionine, and is widely used as a methionine supplement in animal feed formulations.
MMP is produced by the catalytic reaction between acrolein and methyl mercaptan. In a conventional process for the preparation of MMP, liquid acrolein and methyl mercaptan are introduced into a reactor containing liquid phase MMP and a suitable organic base which acts as an olefin/mercaptan addition reaction catalyst. Reaction takes place in the liquid phase. Conventional organic base catalysts for the reaction between acrolein and methyl mercaptan include amines such as pyridine, hexamethylenetetramine and triethylamine. The olefin/mercaptan addition reaction catalyst is typically combined with an organic acid such as acetic acid to inhibit polymerization of acrolein and improve product yield.
HMBN is subsequently produced by the addition reaction between MMP and hydrogen cyanide in the presence of a suitable addition reaction catalyst, which may include the organic bases used to catalyze the reaction between acrolein and methyl mercaptan. Methionine may be produced by reacting HMBN with excess ammonia under high pressure to produce 2-amino-4-(methylthio)butanenitrile and subsequently hydrolyzing the product using a mineral acid to form methionine. Alternatively, methionine may be produced by reacting MMP with ammonium carbonate to form a hydantoin and subsequently hydrolyzing the hydantoin with a base to form methionine. HMBA may be produced by hydrolyzing HMBN using a mineral acid.
Pyridine has proven to be an effective addition catalyst used in preparing both MMP and HMBN. However, it would be highly beneficial to identify effective alternative addition reaction catalysts for the preparation of these valuable intermediates.